Go to Rindir's Staves and purchase a Mage's Hood (or get another piece of weightless head gear). Enchant it with any desired effect you want (a petty soul gem will do fine here), but it is recommended that it be a small, but beneficial enchantment since you do not want to kill the Gray Fox. Rename the enchanted hood anything that starts with two "A"s. When you get to the part of the Thieves' Guild quest line where you take contracts directly from the Gray Fox, reverse-pickpocket the weightless hood into the Gray Fox's inventory. Leave the house and wait a day. When you return, he will have the hood equipped. NPCs normally equip the first item per apparel slot in their inventory, which is usually listed alphabetically. Gray Fox is such an NPC. This will leave the Gray Cowl open for pick pocketing. You will get expelled from the guild, but as long as you wear the Gray Cowl when talking to any guild members, they will not notice.